For the oxidation of reductants in the exhaust gas from the internal or external combustion of fossil fuels, exhaust-gas catalytic converters are utilized, that is to say exhaust-gas aftertreatment devices with a catalytic noble metal coating.
An exhaust-gas catalytic converter, which is subjected to alternating high and low temperatures, of a motor vehicle is subjected to thermal aging, that is to say a degradation of the oxidation efficiency of the catalytic coating owing to noble metal sintering (shrinkage of the surface of the catalytic coating in contact with the exhaust gas owing to the clumping-together of the catalyst material).
The degree of degradation of the oxidation efficiency is dependent (with an Arrhenius or exponential relationship) on the temperature level and is substantially linearly dependent on the exhaust-gas mass flow speed.
The aging of the exhaust-gas aftertreatment device has the result that the light-off temperature of the exhaust-gas aftertreatment device in cold-start phases shifts toward higher temperatures over the course of time, which impairs the capability of the exhaust-gas catalytic converter to convert the reductants emitted by the internal combustion engine during cold-start phases.
In the current systems, it is attempted to increase the exhaust-gas temperature in order to accelerate the increase of the catalytic converter temperature beyond the light-off threshold. Said method leads to an increase in fuel consumption owing to the additional heat which is required from internal or external combustion devices.
The inventors herein have recognized the issues with the above approach and provide an approach to at least partly address them. In one embodiment, a method for controlling an internal combustion engine having an exhaust-gas aftertreatment device during a cold-start phase comprising adjusting a temperature of cooling water to a setpoint operating temperature as a function of a degree of aging of the exhaust-gas aftertreatment device of the internal combustion engine, wherein the temperature of the cooling water is increased more quickly as a present degree of aging of the exhaust-gas aftertreatment device increases.
In this way, a desired level of exhaust pipe cold-start emissions can be maintained with the least possible increase in fuel consumption. The proposed approach is based on limiting the emission of reductants in the engine exhaust gas as a countermeasure against the aging, and the associated rise in light-off temperatures, of exhaust-gas aftertreatment device. This is achieved in that the engine cooling water temperature is controlled as a function of the degree of aging of the exhaust-gas catalytic converter, specifically in such a way that the setpoint operating temperature of the cooling water is reached more quickly with progressive aging of the exhaust-gas catalytic converter.
This has the effect that the thermal losses from the combustion process are reduced, and the gas temperature in the cylinder thus increased, which leads to a reduction of the unburned fuel fraction, and therefore of the reductants, in the exhaust gas.
The advantage of said method is that the energy required to quickly heat the cooling water, which may have a total volume of 4 to 6 liters, to its setpoint operating temperature is significantly lower than the overall energy required to increase the temperature of the inlet gas flow (for example by bypassing the exhaust-gas recirculation cooler or by active heating in the inlet distributor) and/or the temperature of the exhaust-gas flow by use of a later combustion in the cylinder or an external combustion in the exhaust-gas flow.
Furthermore, passive mechanisms such as for example the control of the cooling water circulation in the cooling water tract make it possible to quickly increase the cooling water temperature in the cylinder head. Said cooling water temperature increases very quickly if the cooling water circulation is stopped, either by deactivating the cooling water pump or by control valves in the cooling water circuit.
To accelerate the heating of the cooling water in the case of high degrees of aging of the exhaust-gas catalytic converter, use may furthermore be made of active mechanisms such as for example an electric or microwave heater in the cooling water circuit. The associated effect of the faster heating of the engine oil owing to the coupling to the cooling water circuit through the oil heat exchanger has the secondary effect of reducing engine friction and thus lowering fuel consumption, which in the case of active heating, partially compensates the energy consumption thereof.
The above-described approach permits an optimum compromise between the demands of adhering to both emissions limits and also fuel consumption limits over the operating duration of the internal combustion engine or the service life of the exhaust-gas aftertreatment device.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.